Polypropylene-based multi-layer films are widely used in packaging applications, such as pouches for dry food mixes, pet foods, snack foods, and seeds. Such multi-layer films must have the ability to form reliable hermetic seals at relatively low temperature and, in some instances, the film must do so in the presence of contamination in the seal region, from the contents of the pouches.
U.S. Pat. No. 6,624,247 B1 to Kume et al. (Sumitomo Chemical Company, Ltd.) discloses a polypropylene-based film of a resin composition (C) comprising: 40 to 95 weight percent of a propylene-based copolymer (A) selected from a propylene random copolymer (i) of propylene with ethylene and/or alpha-olefin and a propylene-based copolymer (ii) composed of 1 to 30 weight percent of a component (a) obtained by copolymerizing propylene, an alpha-olefin and ethylene in a first step; and 70 to 99 weight percent of a component (b) obtained by copolymerizing those in the subsequent step, in which the copolymerization ratio is different from that in the first step; and 5 to 60 weight percent of a polypropylene-ethylene and/or alpha-olefin block copolymer (B) having a xylene soluble component (“CXS”) of 5.0 weight percent or more, wherein the CXS has a content of ethylene and/or the alpha-olefin of 14 to 35 molar percent and wherein the heat-seal temperature of the film of the composition (C) is lower by 3° C. (5.4° F.) or more than those of respective films of the compositions (A) or (B).
U.S. Pat. No. 6,641,913 B1 to Hanyu et al. (Fina Technology, Inc.) discloses a multi-layer polyolefin film of the type suitable for packaging application in which heat seals are formed. The multi-layer film comprises a substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110° C. (230° F.). The multi-layer film may be biaxially-oriented. In the production of the multi-layer film, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer is extruded separately to form a surface layer that is thereafter bonded to the interface of the substrate layer at a temperature within the range of 150-260° C.
U.S. Pat. No. 6,534,137 B1 to Vadhar (Cryovac, Inc.) discloses a two-component laminated multi-layer film suitable for use in packaging articles, such as pet food, comprising a first component and a non-heat-shrinkable second component. The first component comprises an outer first film layer, an optional second film layer, and an optional third film layer. The first and third film layers comprise ethylene/alpha-olefin copolymer, while the second film layer is a modified ethylene copolymer. The second component comprises an outer fourth layer, an oxygen barrier fifth layer, sixth and seventh layers which serve as tie layers and are positioned on either side of the barrier layer. The multi-layer film is heat sealable to itself and another film.
U.S. Pat. No. 5,888,648 to Donovan et al. (Mobil Oil Corporation) discloses a multi-layer film which has an improved composite structure for providing hermetic seals to packages manufactured in a high speed packaging apparatus. The structure of the multi-layer film includes a main substrate and a sealant layer. The sealant layer, in turn, includes an intermediate layer which has the primary function of compliance during sealing and a sealing layer which has the primary function of providing adhesivity to the completed seal.
U.S. Pat. No. 6,326,068 to Kong et al. (Mobil Oil Corporation) discloses a multi-layer film that has an improved composite structure for providing hermetic seals to packages manufactured in a high speed packaging apparatus. The structure of the multi-layer film includes layers A/B/C/D. Skin layer A is formed from polypropylene copolymer with melt flow rate greater than one or linear high density polyethylene with melt index greater than one. Core layer B is formed from polypropylene. Intermediate layer C has the primary function of compliance during sealing, and sealing layer D has the primary function of providing adhesivity to the completed seal. The sealing layer D includes an anti-blocking agent comprising non-distortable organic polymer particles having an average particle size greater than 6 microns.
Related U.S. application Ser. No. 10/079,662 to Bader, filed on Feb. 20, 2002, discloses a core layer B that comprises a softening additive blended in a core layer to improve the hermeticity of a sealed package. The softening additive enhances compliance of the core layer with the sealable layer while the seal area is heated under pressure within the crimp jaws during sealing operations. The invention of the '662 application functions during sealing operations to effect a more hermetic seal. The '662 application does not teach effecting a more resilient or durable “post-sealing” seal. The term “compliance” as used in the '662 application is related to non-elastic, deformation or conformance within the sealing jaws during sealing operations due to the improved flowability of the core during heated sealing operation and does not refer to post-sealing seal strength and post-sealing seal performance. It is possible to improve hermeticity as per the '662 application without necessarily, substantially improving minimum seal strength.
Though each of the above films represented a variety of improvements related to packaging films, none of the above films combine desired improvements in processability, seal strength, impact strength, hermeticity, durability, and sufficiently reduced seal temperatures for some of today's challenging packaging operations. Opportunities exist for polymer films to replace other packaging substrates, such as paper and foil, in many temperature-sensitive packaging operations, such as with ice cream bars, chocolate bars, and dry-particulate foods. The present invention meets these and other needs.